The invention has arisen in the course of investigation into improved methods of brazing an inner metal repair tube to an outer metal leaking tube by a heating probe which is entered into the repair tube.
There is currently a problem in the nuclear field, and probably in other fields, where a leak between a tube in a nest of tubes and a tube plate has to be made good without gaining access to the outside of the tube. The practice for dealing with this problem is to insert a short length of repair tube into the leaking tube through the tube plate with one (the outer) end of repair tube explosively welded to the tube plate and the other (the inner) end brazed to the leaking tube.
To effect the braze (which may typically have to be made 10 cms inside a 2.3 cm diameter repair tube) it has further been the practice to construct an induction heating probe which has a water-cooled RF energised coil of about six turns embedded in an insulant. The coil is located to correspond in position with the braze region and when powered, a braze is effected by induced current heating. This practice has created problems of a magnitude justifying a search for improvements.
One problem arises from the bulkiness of the probe. Whilst the coil of the probe is a relatively small item (when used for example with a 2.3 cm diameter tube) it has to carry a transformer which may typically weigh 25 Kg. This is necessary as a large KVA is required (eg 200 volts at 1000 amps) to effect the braze and for power loss reasons this has to be transmitted at lower amperage (eg 100 amps) from an RF generator to a tube repair site. A further problem is the low quality of the braze (as measured by the area at which a bond is effected) and the minimum leak barrier provided by the bond. This is thought to be created in part by uneven distribution of input powers at the braze and haphazard heat exchange relationships whereby the braze alloy can be fused but, by virtue of one of the tubes not receiving adequate heating or being cooled in certain regions, those regions act to solidify the braze alloy without wetting. On the other hand local overheating can give rise to metallurgical problems.
There is also a cooling problem. It is difficult to pass enough water through the water cooled RF energised coil. To meet this problem pre-cooled water is used but this is an inconvenience and expense. There also has to be cooling of a metal sheath which lies above the RF energised coil. This is performed by specially shaping one of the water carry leads to the coil to have a kidney shape in cross-section so that it can be held against the sheath. This is again an inconvenience and expense.